There has been a severe problem with the service life of tool joints and stabilizers used in earth boring such as drilling a bore hole in the earth to a formation or formations from which oil or gas are to be produced since approximately 95 percent of the surface of the earth is composed of silicious materials which are very abrasive and which cause considerable wear on the surfaces of tool joints and stabilizers, as well as on wear prone surfaces of other industrial products.
There have been numerous attempts to provide hardfacing alloys suitable for welding protective hardfacing (referred to as “hardbanding”) on tool joints. For a description of prior art hardbanding for tool joints, reference is made to U.S. Pat. No. 4,256,518, the composite catalog of oil field equipment and services, 1976/77 edition, at pages 3216-19 and pages 4994-5; U.S. Pat. No. 3,067,593. Also, for the use of hardbanding materials, such as tungsten carbide particles to form a hardened surface at a tool joint to increase wear resistance, reference is made to U.S. Pat. No. 3,989,554 issued Nov. 2, 1976 and the history of oil well drilling by J. E. Brantly published in 1971 by the book division of Gulf Publishing Company, Houston, Tex. Also, reference is made to U.S. Pat. Nos. 2,259,232; 2,262,211; 4,431,902; and 4,942,059 which illustrate various prior art ways to hardband tool joints.
Historically, and in practice, tool joints on drilling strings (pipe) such as used in drilling oil and gas wells have been faced at the bottom of the box end with tungsten carbide to resist the abrasion of the rock earth in the drill hole on the tool joint. This has three disadvantages. Tungsten carbide is expensive, it acts as a cutting tool to cut the well casing in which it runs, and the matrix is a soft steel which erodes away easily to allow the carbide particles to fall away.
Most prior art hardfacing materials harder than silicious earth materials are brittle and crack in a brittle manner after solidification and upon cooling due to the brittle nature of its structure and the inability of the structure to withstand solidification shrinkage stresses and typically emit sound energy upon cracking as well as causing considerable casing wear as previously stated. These hardfacing materials are alloys which belong to a well-known group of “high Cr-irons” and their high abrasive resistance is derived from the presence in the microstructure of the Cr-carbides of the eutectic and/or hypereutectic type. In the as-welded condition, whatever the precautions taken, these hardfacing overlays always show a more or less dense network of cracks. Preheat of the base material being hardfaced is not a prerequisite. On the contrary, the lower the preheat and interpass temperatures, the denser the network of cracks which has been considered as a favorable factor from the point of view of the risk of crack propagation into the base material under service conditions.
In the 50 year history of hardbanding tool joints or stabilizers of drilling pipe, no facing which cracked during application to the drilling pipe has been used in practice prior to the development of the invention in U.S. Pat. No. 6,375,895 which includes preheat and post-welding conditions for the hardfacing to withstand abrasive use.
In most industries, however, the metal components which make up the structure and equipment of a given plant must have integrity, which means being free of any kind of cracks since these might be expected to progress through the piece and destroy the part.
When the loss of human life may be involved or when great property damage may result, the requirements for integrity are particularly strict. Examples of such industrial products are pressure vessels in the process industries, structural members in buildings and bridges, and down hole drilling equipment in the oil and gas industry.
Silicious earth particles have a hardness of about 800 Brinell hardness number (BHN). In U.S. Pat. No. 5,244,559 the hardfacing material used is of the group of high Cr-irons that contains primary carbides which have a hardness of about 1700 Hv in a matrix of a hardness of at least 300 BHN to 600 Hv. These primary carbides at this high hardness are brittle, have little tensile strength and hence pull apart on cooling from molten state at a frequency that depends on the relative quantity of the primary carbides in the mix of metal and carbide. Thus, this type of hardfacing material, which is harder than silicious earth materials, when applied by welding or with bulk welding form shrinkage cracks across the weld bead. This material has been applied extensively and successfully during many years for the hardbanding of tool joints and hardfacing of other industrial products. Although the material has become and still is widely accepted by the trade, some users have expressed a desire for a hardbanding tool joint alloy combining the property of minimum possible amount of wear in drill casing with the capability of being welded free of brittle cracks in order to minimize any concerns of mechanical failure risks.
U.S. Pat. No. 6,375,865 describes an alloy having a martensitic-austenitic microstructure from which primary as well as secondary carbides are absent. It is preheated before welding to the industrial product and cooled down after welding.
Wear by abrasion mechanisms always has been, and still remains a main concern in many segments of industry: drilling, mining, quarrying, processing and handling of minerals in general and of highly silicious minerals in particular.
Many base materials and hardfacing alloys have been developed in the past with the aim of achieving the highest possible abrasion resistance compatible with factors such as their decay by mechanical incidences: ruptures and/or spalling.
Typical examples of very highly abrasion resistant hardfacing alloys can be found in the well known family of the high Cr-Irons (high chromium irons), and in particular by a high Cr-Iron such as described in U.S. Pat. No. 5,244,559.
These alloys derive their abrasion resistance properties essentially from their metallographical structure, based on the precipitation of Primary Cr-Carbides. Structures of this type are however affected by a high degree of brittleness and a high sensitivity to cracking when deposited by welding. Hence they exhibit quite a high risk of spalling under actual service conditions. In addition they are not characterized by particularly attractive or low friction coefficients.
Considering the drilling for oil and gas application, in order to reduce the wear induced in the casings by the tool joints, attempts have been made to improve on friction coefficients while maintaining a reasonable and acceptable level of abrasive wear resistance. These efforts have resulted in the development of a Martensitic-Austenitic alloy as described in U.S. Pat. No. 6,375,895 Bi. Alloys of this structural type can be deposited crack-free and are characterized by excellent metal to metal wear properties and low brittleness Hence their susceptibility to spalling is much reduced, however at the expense of a lower resistance against wear by pure abrasion.
The development of the present invention of a new Fe-boron based hardfacing alloy achieves an improved and better optimized balance between abrasive wear resistance and metal to metal wear resistance being particularly advantageous for tool-joint and stabilizer applications.
In order to achieve this goal, an entirely novel approach was needed. As will be described further in more detail, it has consisted in the adoption of Boron instead of Chromium as the element of choice to ensure both the most appropriate microstructures and the required degrees of hardenability.
Years of field experience, gained with the Cr-carbide and with the Martensitic-Austenitic alloys in the field of oil and gas drilling, have demonstrated both its advantages and shortcomings.
Field testing, conducted on tool-joints of the Fe-boron alloy of the present invention, under conditions of drilling through various geological mineral formations from extremely abrasive to lesser abrasive ones has demonstrated beyond any doubt that the new approach which was adopted for the hardfacing alloys of the present invention has enabled the full accomplishment of the above outlined goal consisting in the production of a hardfacing alloy capable of achieving an exceptional combination of both high metal to metal wear resistance and extreme resistance to abrasion. In this perspective, the field test results fully confirm the results of extensive preliminary laboratory testing of the Fe-boron hardfacing alloy of the present invention.
The properties of Boron as an alloying element have been exploited in a number of cases for achieving different objectives. These cases are however limited to low-alloyed carbon-steel weld metals in which boron is used often in conjunction with Titanium to promote the formation of a particular type of phase known as acicular ferrite. The aim there was to achieve improved impact properties. The Boron additions were limited to a few parts per million (ppm).
Also, in high Carbon high alloyed Fe-based hardfacing weld metals, Boron has been used at levels of about up to 1 percent with the objective of strengthening the matrixes by induction of martensitic types of transformation, but in all cases at the expense of a significant increase of the brittleness of the alloys especially when boron additions are combined with high carbon contents.
A significant difference, which has important practical implications, is that the hardfacing alloy of the present invention contains primary borides and a low carbon content characterized by a quadratic crystallographic structure which while having cracks functions essentially as crack-free hardfacing in use; whereas, the primary Cr-carbides solidify with a form of a hexagonal structure and a higher Carbon content and which when welded in place have brittle cracks.
It would be highly desirable and advantageous to provide a hardfacing alloy composition and industrial products hard surfaced with it capable of having the exceptional combination of both high metal to metal wear resistance and extreme resistance to abrasion such as by hardbanded on tool joints and stabilizers as well as other industrial products.